Processes for the production of ethyl acrylate by interacting acrylic acid with ethylene in the presence of sulfuric acid are well known; see for example, U.S. Pat. No. 3,703,539, issued Nov. 21, 1972 to DiLiddo; U.S. Pat. No. 3,539,621, issued Nov. 10, 1970 to Cipollone et al; and U.S. Pat. No. 3,894,076, issued July 8, 1975 to Van Duyne et al. As described in these and other references, the reaction is believed to involve the formation of intermediate sulfates from the reaction of ethylene with sulfuric acid. These sulfates further react with acrylic acid to form ethyl acrylate. To provide a product in good overall yields with high carbon efficiencies, unreacted ethylene, acrylic acid and the sulfuric acid residue must be recycled to the reactor. The residence times of some of these reactants can be long enough to result in the formation of partially polymerized products which can plug the process equipment. Additionally, these processes operating on a continuous basis for extended periods of time produce impurities in the reaction system which must be efficiently removed.
A method which has been used to remove impurities formed in the ethylene-acrylic acid reaction to ethyl acrylate is to pass a minor portion of the sulfuric acid residue recycled to the ethylene-acrylic acid reactor to a wiped-film evaporator whose initial stages are heated to enable the sulfuric acid residue to reach a temperature in the range from about 300.degree. F. to about 360.degree. F. and whose last or withdrawal section(s) are cooled to enable the sulfuric acid residue to reach a temperature in the range from about 230.degree. F. to about 280.degree. F. The spent sulfuric acid residue is thereby removed from the reaction system.
In the chemical industry, it is common to use a wiped-film evaporator for processing highly viscous solutions and for vacuum evaporation. Typical of the units that can be used and are known are the types described in a three part report in Chemical Engineering, Sept. 13, 1965, pages 175-190, entitled "Agitated Thin-Film Evaporators". Modifications of these types of evaporators are described in the art, for example, in U.S. Pat. No. 3,292,683 entitled "Wiped Falling Film Evaporator" issued Dec. 20, 1966 to Buchi et al; U.S. Pat. No. 3,464,478 entitled "Horizontal-Type High Vacuum Film Evaporator For Highly Viscous Solutions" issued Sept. 2, 1969 to Tomoharu Ueda et al; U.S. Pat. No. 3,695,327 entitled "Wiped Thin Film Evaporation and Treatment Process" issued Oct. 3, 1972 to Widmer; among others. This equipment is normally used to separate highly viscous materials from volatile materials with the objective to separate the materials at the highest efficiencies in the least amount of time. Under these conditions, these evaporators are maintained at the highest possible temperatures for the most efficient periods of time. None of the processes and equipment known in the use of wiped-film evaporators area are known to evaporate and cool within the same unit. In the process of producing ethyl acrylate by the reaction of ethylene and acrylic acid in the presence of sulfuric acid, it is the technique of heating the initial stages of the evaporator and cooling the last or withdrawal section(s) which provides the highly efficient and desirable process of separating the highly viscous sulfuric acid residue from the volatile products such as ethyl acrylate and acrylic acid while maintaining foaming of the evaporating material at a workable and controllable minimum.
Cooling the last or withdrawal section(s) of the wiped-film evaporator substantially eliminates foaming of the spent sulfuric acid residue. If the sulfuric acid residue is not cooled, foaming of the residue in the wiped-film evaporator can be sufficiently severe that the foam can back through the lines leading to the wiped-film evaporator leading to a shut down of the wiped-film evaporator. This results in significant acrylic acid and ethylene efficiency losses and can lead to total process shut down if adequate sulfuric acid purge is not withdrawn from the system. Furthermore, if various impurities are not removed from the system, increased polymer formation can occur in the ethylene-acrylic acid reactor which can lead to plugging of the process equipment. It has been discovered that significant amounts of polymerization initiators produced in the ethyl acrylate production can be removed from the reaction process by passing at least 50 weight percent of the ethyl acrylate product residue separated from the substantially pure ethyl acrylate product through the same wiped-film evaporator unit used for the sulfuric acid residue.